How can i create a seamless blend between two objects and give the illusion of rubber stretching?
Add a plane with some subdivisions: hit W then S (in 2.79) a few times. Add a cube above it, adjust its scale then subdivide it a few times. Delete the faces at the legs in the cube and in the plane (one face/leg). Select the edges around the holes both on the cube and the floor, then bridge them with "Bridge edge loops" command. Then add a "Subdivision surface" modifier to the mesh. You can correct the legs with new edge loops by hitting ctrl+p.